A pressure sensor having a configuration in which strain gauges are provided on a diaphragm is well known. This pressure sensor detects a pressure by using a change in resistance of the strain gauges based on deformation of the diaphragm caused by a pressure. Generally, four strain gauges constitute a bridge circuit, and a differential voltage output that is in proportion to a pressure is obtained from this bridge circuit to detect the pressure. Herein, the bridge circuit is mainly used for temperature compensation. This is because, in the case where the four strain gauges are similarly changed, output of the bridge circuit is not changed even when the strain gauges have temperature characteristics.
For a low pressure equal to or lower than about 1 MPa, there is frequently used a pressure sensor having a configuration in which a silicon diaphragm obtained by processing a silicon substrate to form a diaphragm is provided. PTL 1 discloses that a thin diaphragm is formed by etching a back surface of a silicon substrate and strain gauges made of polycrystal silicon are formed on the diaphragm. According to PTL 1, when two of four strain gauges are arranged in a peripheral portion of the diaphragm and the other two strain gauges are arranged in a central portion of the diaphragm, stresses generated in the strain gauges by application of a pressure become a compressive stress and a tensile stress. PTL 1 further discloses that, when the diaphragm has a rectangular shape, a stress in a vertical direction which negatively effects on sensor sensitivity can be reduced in the strain gauges arranged at the center of the diaphragm.
For a high pressure equal to or higher than about 1 MPa, or when, for example, silicon cannot be exposed because corrosion resistance is needed, there is frequently used a pressure sensor having a configuration in which a diaphragm made of stainless steel is provided and semiconductor strain gauges are attached on the diaphragm or a sensor chip on which semiconductor strain gauges are provided is attached thereon, PTL 2 discloses that a sensor chip constituted by a single crystal semiconductor in which strain gauges are provided is bonded onto a circular diaphragm made of metal. Note that a size of the sensor chip is larger than that of the diaphragm and the sensor chip is bonded so that the strain gauges on the sensor chip are arranged in a peripheral portion of the diaphragm. According to PTL 2, when two of four strain gauges are arranged in a circumferential direction and the other two strain gauges are arranged in a radial direction, stresses generated in the strain gauges by application of a pressure become a compressive stress and a tensile stress, and thus sensor sensitivity is obtained. PTL 2 further discloses that, when the sensor chip has a polygonal shape that is as close to a circular shape as possible, a bad influence of a thermal stress caused by a difference between coefficients of thermal expansion of the sensor chip and the diaphragm can be removed as much as possible.